The present invention relates to a circuit for maintaining a laser operating relatively efficiently. More specifically, the present invention relates to such a circuit using a thermoelectric cooler.
The amplitude of light out of a laser diode is reduced as the diode heats up. Additionally, some systems have problems if laser diodes change wavelength due to heating. Accordingly, various arrangements including heat sinks and/or cooling fans have been used to prevent such semiconductor lasers from having their light output amplitude overly reduced by heat or having the wavelength of output light changed too much. If a laser diode is used for an optical transmitter, temperature change problems can adversely affect the operation of the transmitter (which may be part of an optical transceiver). The following patents are among various prior designs:
______________________________________ U.S. Pat. No. Inventor Issue Date ______________________________________ 4,288,854 Burroughs September 8, 1981 4,433,238 Adolfsson et al February 21, 1984 4,583,228 Brown et al April 15, 1986 4,733,253 Daniele March 22, 1988 5,029,311 Brandkamp et al July 2, 1991 5,043,992 Royer et al August 27, 1991 5,140,607 Paiva August 18, 1992 ______________________________________
The Daniele patent discloses feedback control of temperature in a laser diode and the use of a thermoelectric cooler 70 in a feedback arrangement for a diode 15 in FIG. 3.
The Burroughs patent discloses a temperature controller for controlling a body which is disclosed as being an injection laser for use in an optical communication system.
The Adolfsson patent discloses at the top of column 6 that a thermoelectric cooler may be used to maintain a light emitting diode at a constant voltage.
The Brown patent discloses frequency stabilization of lasers using a feedback arrangement.
The Brandkamp patent discloses a fluorescent lamp having a thermoelectric cooler 81 to stabilize temperature. The lamp is used in a document scanning system.
The Royer patent shows a laser driver having temperature compensation.
The Paiva patent discloses a laser device with an arrangement for dissipating and stabilizing heat generated by the laser diode.
Thermoelectric coolers have been used for cooling lasers, but have generally required larger, more expensive power supplies than would otherwise be used and/or such coolers are connected in such a way that a significant amount of power is wasted.
FIG. 1 shows a well known circuit for powering an optical transceiver system 10 by a voltage source 12. The transceiver system 10 includes a voltage regulator 14 and the other transceiver components simply shown as block 16. In well known fashion, the voltage regulator 14 sends a small amount of current down line 18 to bypass block 16, this allowing the voltage across block 16 to be maintained constant or essentially constant. The resistance or impedance between lines 20 and 22 is adjusted by the regulator such that the voltage drop across lines 20 and 22 of the regulator may vary to compensate for variations in source 12, thus maintaining the voltage across 16. The power used by the voltage regulator is wasted in the sense that it is simply expended to protect the useful components of block 16 from any adverse effects of supply fluctuations.
Assuming that the block 16 included a laser diode (not separately shown) and cooling of the diode is to be made using a thermoelectric cooler, FIG. 2 shows a known way of attaching the same source 12 to a second voltage regulator 24 which regulates power supplied to a thermoelectric cooler 26. The thermoelectric cooler is in thermal contact with the unshown laser which is being cooled. Again, the power used by the voltage regulator 24 is wasted in the sense that it is simply expended to stabilize the voltage across useful component thermoelectric cooler 26. Since the source 12 often is 12 volts and the thermoelectric cooler 26 should have for example about 1 volt maintained across it, the circuit of FIG. 2 wastes more energy than is put to good use.
FIG. 3 is a simplified perspective showing how the prior art includes a structure having a laser diode 28 with a monitor photodiode 30, thermoelectric cooler 32, and thermistor 34 sold as a unit within or on housing 36.
A known or prior art way of hooking up the FIG. 3 components is shown in simplified form in FIG. 4. The photodiode 30 receives a portion of the output of laser 28, which output is stabilized by feedback laser drive circuit 38 and reference adjust variable resistor 40. A temperature feedback circuit 42 uses thermistor 34 and reference adjust resistor 44 to provide feedback stabilization of the laser diode temperature. For ease of illustration, FIG. 4 doesn't show the voltage regulator and various lead and/or lag networks which would be used for improving operation.